NEW NUMERICAL TOOL FOR INVERSE AND FORWARD MODELING OF COMPLEX GEOLOGICAL STRUCTURES: APPLICATION TO THE ANALYSIS OF A SANDBOX EXPERIMENT

We present a new geomechanical computer tool based on the finite element technique, which simulates the behavior of complex geological structures such as folded and faulted rock. The model undergoes infinitesimal or finite linear elastic deformation and is composed of a heterogeneous, anisotropic medium. Inelastic deformation is accommodated by discontinuities (faults and fractures). Applications of this new tool include the forward analysis of tectonic folding and faulting in sedimentary basins and mechanically-based 2D and 3D structural restoration.

To demonstrate the capability of the new tool we analyzed fault development in the hanging wall of a syn-sedimentary listric normal fault. Because the complete deformation history is known, an example was taken from one of the sandbox experiments carried out by McClay (1990). A model of the final deformed stage of this analogue model was created and restored sequentially by removing the upper sedimentary layers one by one. The top of the next upper sedimentary layer was constrained to be horizontal while the base of the model was constrained to follow the shape of the listric basal fault. The faults were constrained to stay in contact and for simplicity to have zero friction. The elastic properties were homogeneous throughout the model. For each step the faults were free to accommodate any slip until the model equilibrated (SF=0) and the elastic deformations were minimized. We mapped the picture of the final stage of the sandbox experiment onto the numerical model grid in order to follow the deformation of the layers during each step of the restoration. The model also was analyzed in a forward sense in order to sequentially investigate the development of the faults in the hanging wall of the basal listric fault. The chronology of the numerical fault development is compared to the chronology inferred from the analysis of sedimentation thickness variations and both fault geometry and slip distribution are described.

The numerical model corresponds well to the physical model and provides additional insights about the physics of the process and quantitative values of physical parameters. We conclude that this tool has significant potential for analyzing physical models and natural examples of complex geological structures.